Epitaxial Ni_xPd_1-x (111) Alloy Substrates with Continuously Tunable Lattice Constants for 2D Materials Growth

Epitaxial strain can be a powerful parameter for directing the growth of thin films. Unfortunately, conventional materials only offer discrete choices for setting the lattice strain. In this work, it is demonstrated that epitaxial growth of transition metal alloy solid solutions can provide thermally stable, high-quality growth substrates with continuously tunable lattice constants. Molecular beam epitaxy was used to grow Ni_xPd_1-x(111) alloy films with lattice constants between 3.61 and 3.89 Å on the hexagonal (0001) basal planes of α-Al₂O₃ and Cr₂O₃ (grown as epitaxial films on α-Al₂O₃ (0001)). The Cr₂O₃ acted as an adhesion layer, which not only improved the high-temperature stability of the films but also produced single-domain films with Ni_xPd_1-x [112̅] parallel to Cr₂O₃ [112̅0], in contrast to growth on α-Al₂O₃ that yielded twinned films. Surface characterization by electron diffraction and scanning tunneling microscopy (STM) as well as bulk X-ray diffraction analysis indicated that the films are suitable as inexpensive and stable substrates for thin-film growth and for surface science studies. To demonstrate this suitability, bilayer SiO₂, a two-dimensional van der Waals material, was grown on a Ni_xPd_1-x(111) film tuned to closely match the film's lattice constant, with STM and electron diffraction results revealing a highly ordered, single-phase crystalline state.